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Understanding Disk Arrays: FastTrak100™ Ultra ATA/100 RAID Card User's Guide

General Description • Disk Array Member • Disk Array Types • Striping (RAID 0) • Mirroring (RAID 1)

General Description

A "disk array" is formed from a group of 2 (or more) disk drives which appear to the system as a single drive. The advantage of an array is to provide better throughput performance and/or data fault tolerance. Better performance is accomplished by sharing the workload in parallel among multiple physical drives. Fault tolerance is achieved through data redundant operation where if one (or more) drive fails or has a sector failure, a mirrored copy of the data can be found on another drive(s). For optimal results, select identical Ultra ATA/100 drives to install in disk arrays. The drives' matched performance allows the array to function better as a single drive.

Disk Array Member

The individual disk drives in an array are called "members." Each member of a specific disk array is coded in their "reserved sector" with configuration information that identifies the drive as a member. All disk members in a formed disk array are recognized as a single physical drive to the system.

Disk Array Types

For most installations, the FastBuildTM setup "<1> Auto Setup" option will configure your system. There are two disk array types that can be installed using the FastTrak100 card supplied by Dell. Striping is in the Performance category while Mirroring is in the Fault Tolerance category. Disk arrays within the Performance and Fault Tolerance categories conform with the Redundant Array of Independent Disks technology, or RAID. Table 1 below shows differences of these RAID levels by performance, storage capacity, and number of drives supported.

Table 1. RAID level differences

Striping (RAID 0)

Reads and writes sectors of data interleaved between multiple drives. When any disk member fails, it affects the entire array. Performance is better than a single drive since the workload is balanced between the array members (see Figure 1 below). This array type is for high performance systems. Identical drives are recommended for performance as well as data storage efficiency. The disk array data capacity is equal to the number of drive members times the smallest member capacity. For example, one 1GB and three 1.2GB drives will form a 4GB (4 x 1GB) disk array. Stripe Size - a value can be set from 1KB to 1024KB sector size. The size can directly affect performance. In the FastBuild BIOS, the "Desktop" default is 8KB while "Server" and "A/V Editing" are 64KB.

Figure 1. RAID 0 striping

Mirroring (RAID 1)

RAID 1 mirroring writes duplicate data on to a pair of drives (see Figure 2 below) while reads are performed in parallel . ATA RAID 1 is fault tolerant because each drive of a mirrored pair is installed on separate IDE channels. If one of the mirrored drives suffers a mechanical failure (e.g. spindle failure) or does not respond, the remaining drive will continue to function. This is called Fault Tolerance. If one drive has a physical sector error, the mirrored drive will continue to function.

On the next reboot, the FastBuildTM utility will display an error in the array and recommend to replace the failed drive. Users may choose to continue using their PC, however Promise recommends replacing the failed drive as soon as possible.

Due to redundancy, the drive capacity of the array is half the total drive capacity. For example, two 1GB drives that have a combined capacity of 2GB would have 1GB of usable storage. With drives of different capacities, there may be unused capacity on the larger drive.

Figure 2. RAID 1 mirroring

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